1. Field of Invention
The present invention relates to a continuous heat treatment furnace used for heat treatment of substrates of solar batteries, etc. and a heat treatment method using the furnace.
2. Description of the Related Art
The method of making substrates of solar batteries includes steps of printing an electrically conductive electrode paste on a surface side and a back side of a substrate at a predetermined pattern, and then heat treatment of the substrate (removal of binder, drying and firing) by continuously or intermittently moving the substrate in a continuous heat treatment furnace. Usually, in the heat treatment furnace used for such heat treatment, a drying and binder-removing area where drying of the objects to be thermally treated and/or removal of binder are carried out and a firing area where the substrate is fired are provided in succession from inlet side towards outlet side of the furnace, and the objects to be thermally treated are subjected to drying and/or removal of binder while being transported through the drying and binder-removing area and thereafter fired while being transported through the firing area, and thereafter taken out from the furnace.
As the transporting mechanism for transporting the objects to be thermally treated in a heat treatment furnace, a mesh belt conveyor is widely used when the objects to be thermally treated are substrates of solar batteries (see, for example, Patent Document 1). Furthermore, recently, walking beams or transporting mechanism for conducting a transporting operation like that of walking beams by wire materials such as wires stretched by applying a tension are also employed because they are smaller in heat capacity than the mesh belt conveyors and rapid increase and decrease of temperature can be performed (see, for example, Patent Document 2).
The firing of substrates of solar batteries after drying and/or removal of binder, namely, baking of an electroconductive paste comprising aluminum or silver onto the surface of the substrates is ideally carried out by rapidly heating to about 800° C. in a short time and then rapidly cooling the substrates for obtaining satisfactory product characteristics. In order to attain this ideal firing state, it is required to increase the transporting speed of the objects to be thermally treated in the firing area than the transporting speed in the drying and binder removing area, thereby to rapidly pass the objects in a short time through the firing area maintained at a high atmospheric temperature of about 1000° C.
However, the conventional continuous heat treatment furnaces have such a construction that a series of heat treatments of drying and/or removal of binder and firing are performed while transporting the objects to be thermally treated at the same speed in all of the drying and binder removing area and the firing area by one kind of transporting mechanism. Thus, it is very difficult to attain the above-mentioned ideal firing state.
Furthermore, for making uniform the product characteristics of the objects to be thermally treated, it is important to maintain the firing area stably at a constant high temperature. However, when a transporting mechanism of high heat capacity, such as mesh belt conveyor, is used, the change of temperature in the firing area caused by movement of the transporting mechanism is great and a stable temperature in the firing area cannot be obtained. Moreover, it is important for obtaining good product characteristics that the transporting of the objects to be thermally treated is not only performed rapidly in a short time, but also performed so that a necessary heating time is ensured and the total heating time of the objects becomes nearly uniform.
Patent Document 1: Japanese Patent Laid-open No. 203888/2002
Patent Document 2: Japanese Utility Model Publication No. 4470/1995